timing based execution selection and execution

clear all;

close all;

clc;

t=1:1000;

fm=4000;

fs=80000;

msg=sin(2*pi*fm*t); %%% I want this signal for for 1000ms

a1=[100,200,300;0,150,0;250 350 400];

[m,n]=size(a1);

for i=1:m

for j=1:n

if a1(i,j)>0 % this loop i want to continue for 100ms and after 100ms i want that i,j value should be changed (a1 value) but msg should continue from were it has finished 100ms

result(i,j)=msg*cos(2*pi*a1(i,j)*t);

else

result(i,j)=0

end

end

end